Philip L. Varghese

Approach for Modeling Rocket Plume Impingement and Dust Dispersal on the Moon

When a lander approaches the lunar surface, the plume from the descent engine impinges on the ground and entrains loose regolith into a high-velocity spray. This problem is simulated with a hybrid ...

Quasi-state-specific QCT method for calculating the dissociation rate of nitrogen in thermal non-equilibrium

The dissociation of nitrogen was studied using a quasi-classical trajectory (QCT) analysis in the context of calculating the dissociation rate surface for a dense range of temperatures for use in computational fluid dynamics (CFD) applications. By sampling rovibrational states from a Boltzmann distribution but uniformly sampling the relative speed, the dissociation rate was calculated for translational and rovibrational temperatures between 8000 K and 20000 K. The justification for this approach was verified by analyzing different sampling techniques. It was found that uniformly sampling the relative speed increased the uncertainty of the thermally averaged dissociation rate, but the same QCT results could be used for a large range of temperatures. This is in contrast to Monte Carlo sampling techniques, where a new batch of trajectories must be simulated for each desired temperature. To generate the dissociation rate surface, 500 million trajectories were simulated, and the non-equilibrium rates were compared to other models and experimental data, generally showing good agreement.

Sensitivity Analysis of DSMC Parameters for Ionizing Hypersonic Flows

This work investigates the sensitivity of direct simulation Monte Carlo (DSMC) input parameters for a high temperature hypersonic flow scenario. The DSMC model simulates a rarefied hypersonic shock tube experiment with air. A previous model has been improved to include 11-species air with charged species collisions and reactions in order to model high temperature interactions appropriately. The DSMC code is ultimately to be linked with the HPC-Rad radiative transport code and results are to be compared with the NASA Electric Arc Shock Tube (EAST) data for a peak radiative heating lunar return scenario. A global Monte Carlo sensitivity analysis is conducted on this scenario to investigate which reaction rates have the greatest effect on the simulation results. The sensitivity of each reaction rate was measured by calculating the square of the Pearson correlation coefficient and the mutual information for a certain quantity of interest (QoI). The most sensitive parameters are identified in preparation for future Markov Chain Monte Carlo calibrations with the EAST data.

Vibrational non-equilibrium effects in supersonic jet mixing

A joint experimental and computational study is being conducted to investigate the effects of vibrational non-equilibrium on supersonic combustion, although the focus of this paper is on mixing between a supersonic jet and a subsonic coflow. A new facility has been constructed that consists of a Mach 1.5 turbulent jet issuing into an electrically heated coflow. In the preliminary experiments reported here, air is used in both the jet and the coflow. The degree of non-equilibrium in the jet shear layers is quantified by using high-spectral resolution timeaverage spontaneous Raman scattering. The Raman scattering is complemented with planar temperature imaging using Rayleigh scattering. Much of the current work is focused on the extent to which vibrational non-equilibrium can be assessed by using time-averaged Raman scattering in a turbulent flow with large-scale temperature fluctuations. The experimental work is supported by direct numerical simulation of related jet flows. Preliminary DNS of turbulent jets in coflow with imposed vibrational non-equilibrium shows that vibrational relaxation effects have a first-order effect on the jet temperature field and mixing physics.

Spontaneous Raman Scattering Temperature Measurements and Large Eddy Simulations of Vibrational Non-equilibrium in High-Speed Jet Flames

High-speed turbulent diffusion flames were investigated using time-averaged spontaneous Raman scattering to determine the vibrational and rotational temperature of the major diatomic species, N2 and O2. Mixing-induced thermal non-equilibrium is detected in the shear layer upstream of the turbulent hydrogen flame in N2 molecules but not O2. Rotational temperatures of the two species agree to within the measurement precision. The non-equilibrium is relaxed immediately beyond the average flame-base location due to the presence of combustion products. The non-equilibrium measured in a lower speed methanehydrogen flame is significantly weaker, as expected. The presence of non-equilibrium is confirmed using Rayleigh thermometry images to quantify the effect of translational temperature variation in the Raman measurement volume. The effect of interspecies vibrational energy transfer is investigated using large-eddy simulations of the experimental flow. Good agreement is found between the measurements and average simulated temperature fields when the interspecies vibrational coupling is very weak.

Discrete velocity computations with stochastic variance reduction of the Boltzmann equation for gas mixtures

We extend a variance reduced discrete velocity method developed at UT Austin [1, 2] to gas mixtures with large mass ratios and flows with trace species. The mixture is stored as a collection of independent velocity distribution functions, each with a unique grid in velocity space. Different collision types (A-A, A-B, B-B, etc.) are treated independently, and the variance reduction scheme is formulated with different equilibrium functions for each separate collision type. The individual treatment of species enables increased focus on species important to the physics of the flow, even if the important species are present in trace amounts. The method is verified through comparisons to Direct Simulation Monte Carlo computations and the computational workload per time step is investigated for the variance reduced method.

A low noise discrete velocity method for the Boltzmann equation with quantized rotational and vibrational energy

Abstract A discrete velocity method is developed for gas mixtures of diatomic molecules with both rotational and vibrational energy states. A full quantized model is described, and rotation–translation and vibration–translation energy exchanges are simulated using a Larsen–Borgnakke exchange model. Elastic and inelastic molecular interactions are modeled during every simulated collision to help produce smooth internal energy distributions. The method is verified by comparing simulations of homogeneous relaxation by our discrete velocity method to numerical solutions of the Jeans and Landau–Teller equations, and to direct simulation Monte Carlo. We compute the structure of a 1D shock using this method, and determine how the rotational energy distribution varies with spatial location in the shock and with position in velocity space.

DSMC Shock Simulation of Saturn Entry Probe Conditions

This work describes the direct simulation Monte Carlo (DSMC) investigation of Saturn entry probe scenarios and the influence of non-equilibrium phenomena on Saturn entry conditions. The DSMC simulations coincide with rarefied hypersonic shock tube experiments of a hydrogen-helium mixture performed in the Electric Arc Shock Tube (EAST) at the NASA Ames Research Center. The DSMC simulations are post-processed through the NEQAIR line-by-line radiation code to compare directly to the experimental results. Improved collision cross-sections, inelastic collision parameters, and reaction rates are determined for a high temperature DSMC simulation of a 7-species H2-He mixture and an electronic excitation model is implemented in the DSMC code. Simulation results for 27.8 and 27.4 km/s shock waves are obtained at 0.2 and 0.1 Torr, respectively, and compared to measured spectra in the VUV, UV, visible, and IR ranges. These results confirm the persistence of non-equilibrium for several centimeters behind the shock and the diffusion of atomic hydrogen upstream of the shock wave. Although the magnitude of the radiance did not match experiments and an ionization inductance period was not observed in the simulations, the discrepancies indicated where improvements are needed in the DSMC and NEQAIR models.

Numerical investigation of vibrational relaxation coupling with turbulent mixing

In flows where the relaxation rate of vibrational motion of the molecules to equilibrium is comparable to the flow through time scales, the presence of turbulence can alter the mixing and equilibration process. To understand the coupling between mixing and vibrational relaxation, a novel state-specific species model is solved in a background turbulent flow. The method is applied to mixing of two nitrogen streams at different static temperatures. The relaxation rates for each state are computed using quasi-classical trajectory analysis. For the flow conditions considered, the first ten vibrational levels are computed in the flow solver.The direct numerical simulation shows that population in different vibrational levels are significantly affected by turbulence and that the local distribution becomes nonBoltzmann. In certain locations in the jet, the population from the direct calculation can be several orders of magnitude different than the local-temperature based Boltzmann level. Last, while the bulk vibrational energy is inferior to its local equilibrium value throughout the mixing layer, the high energy level populations (levels 3 to 8) are on the opposite always over-populated. As chemical reactions are affected by these high vibrational energy populations, a simple temperature model would under-estimate the impact of nonequilibrium on combustion.

Lunar Dust Transport Resulting from Single- and Four-Engine Plume Impingement

When the exhaust plume from a descent engine impinges on the lunar surface, loose regolith can erode and become entrained into a high-velocity spray. These processes are simulated in this work by several integrated models: a hybrid continuum–kinetic solver for the gas flowfield, a coupled two-phase flow model for a polydisperse distribution of grain sizes, and a model for inelastic grain–grain collisions. The continuum regime is modeled with the data-parallel line relaxation code, and the kinetic modeling is done via the direct-simulation Monte Carlo method. Simulation results are first presented for a single-engine lander hovering at different altitudes. Surface stresses and the resulting dust erosion are compared to classical theory, and correction terms are introduced to improve agreement. The velocities of different-sized particles and particle mass fluxes are shown for different hovering altitudes. For a four-engine lander, there are multiple plume–plume and plume–surface interactions that result in ...